A current analog television such as an LCD TV or a plasma TV, in order to display video signals like composite video broadcast signals (CVBS) on a screen, requires horizontal and vertical synchronization signals. FIG. 1 shows the structure for generating horizontal synchronization signals by a prior analog television. A slicer 11 receives a CVBS signal, and produces a pulse signal when the CVBS signal crosses a specific voltage level from high to low to indicate that a horizontal synchronization signal in the CVBS signal is detected. The pulse signal is then forwarded to a phased lock loop (PLL) 10, in which a phase detector 12 detects a phase difference between the pulse signal and a horizontal synchronization signal generated by a numerically controlled oscillator (NCO) 14. Based on the phase difference, a loop filter 13 calibrates the phase of the horizontal synchronization signal generated by the NCO 14.
With reference to FIG. 2 showing a timing diagram of the signals in FIG. 1, an ideal timing point of the pulse signal 11 produced by the slicer 11 is a falling edge of the horizontal synchronization signal in the CVBS signal. The NCO 14 has a counter, which has a counter value thereof incrementing with time. When the counter value reaches a certain upper limit, the counter resets the counter value to zero to repeat the incrementing process, as well as to produce a pulse signal for the horizontal synchronization signal. Referring to FIG. 2, the figure shows a phase difference between the horizontal synchronization signal generated by the NCO 14 and the pulse signal produced by the slicer 11. Based on the phase difference, the loop filter 13 calibrates the phase of the NCO 14 in order to eliminate the phase difference.
However, during a transmission process of video signals, a horizontal synchronization signal in a video signal is inevitably distorted due to channel effects, rising to misjudgment of the slicer 11. For example, referring to FIG. 3A, because of distortion of the horizontal synchronization signal in the CVBS signal, the slicer 11 misjudges falling edges thereof to P3 and P4 instead of the ought-have-been P1 and P2. As a result, the phase difference detected by the phase detector 12 gets even larger. At this point, suppose the loop filter 13 calibrates the NCO 14 by a large range, the phase of the horizontal synchronization signal output by the NCO 14 is consequently distorted such that a picture displayed jitters. Therefore, to avoid the picture from jittering, the NCO 14 is only allowed with calibration by a small range with regard to the considerably large phase difference detected by the phase detector 12.
Further, the phase detector 12 may also detect a large phase difference under other circumstances. Referring to FIG. 3B showing changes in the CVBS signal forwarded to the slicer 11 when a television channel is switched, a signal from the original channel not yet completely processed is switched to a signal from the new channel. Nevertheless, the horizontal synchronization signal output from the NCO 14 is unable to correspondingly change right away, and so a quite large phase difference is detected by the phase detector 12. Suppose the NCO 14 calibrates by a small range, a relatively long adjustment period is needed after switching channel for the picture to restore to normal, hence undesirably affecting a user's visual effects.
Conclusive from the foregoing description, the prior structure in FIG. 1 has large phase differences when encountering the two circumstances listed above and as shown in FIGS. 3A and 3B. Jitter is much likely resulted owing to unsatisfactory calibration of the phase of the horizontal synchronization signal.